Nowadays various types of display devices using organic fluorescent materials (organic luminescent devices) are actively researched and developed. Of these devices, much attention is focused on organic electroluminescent (EL) devices. This is because organic EL devices are promising display devices capable of emitting light of high luminance under a low applied voltage. For instance, the EL devices of a type which comprises organic thin layers formed by evaporating organic compounds are known (Applied Physics Letters, vol. 51, p. 913 (1987)). More specifically, the organic EL devices of such a type have a laminated structure made up of an electron transfer material and a hole transfer material, and their luminous characteristics show substantial improvements over those of conventional devices of single-layer type.
With the reports printed in the journal described above, the study and development of organic EL devices have been made energetically. And developments of electron transfer materials and hole transfer materials have been attempted with the intention of enhancing luminous efficiency. As to the development of electron transfer materials, however, no compounds superior in properties to tris(8-hydroxyquinolinato)aluminum (usually abbreviated as "Alq") have yet been found. Such being the case, it has been desired to develop compounds capable of surpassing Alq in properties. In addition, Alq fluoresces a green color, so that it has no suitability as an electron transfer material for blue luminescent devices Therefore, it has been desired to find out electron transfer materials suitable for blue luminescent devices.
Also, the application of organic EL devices to full color display has been lively examined in recent years. In order to develop a high-performance full color display, it is necessary to heighten the color purity of each of blue luminescence, green luminescence and red luminescence. However, the luminescence of high color purity is difficult to obtain. For instance, the distyrylarylene compounds (DPVBi) described in a book, entitled Yuki EL Soshi to sono Kogyoka Saizensen (which means "Organic EL devices and the forefront of their industialization"), page 38, published by N.T.S. Co., and Zn(OXZ).sub.2 (benzene ring-condensed nitrogen-containing heterocyclic compounds) described in the book, supra, page 40, and JP-A-7-133483 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") are blue luminescent materials which have undergone extensive examinations, but they can merely provide blue luminescence of low color purity. Therefore, there is plenty of room for improvement.
Another important characteristic that is required for organic EL device materials is durability. In particular, the amorphous film stability constitutes an important factor in the enhancement of durability. Therefore, it has been expected to develop compounds usable as organic EL device materials and capable of forming highly stable amorphous films. For instance, N,N'-diphenyl-N,N'-di(m-tolyl)benzidine (TPD) is an extensively utilized hole transfer material and has high hole transfer capacity. Although the TPD evaporated film can be in a uniform amorphous state for a short while after the evaporation was finished, cases occurs in which the evaporated film crystallizes after a lapse of several hours. In such cases, the durability of EL devices is greatly lowered.